The present invention relates to modification of a film surface to enhance the adhesive characteristics of the film. More particularly, the present invention relates to a process for surface modification of oriented polymeric film by application of a short pulse of intense, broad spectrum electro-magnetic radiation thereto.
Biaxially oriented polyethylene terephthalate film is generally considered impossible to heat seal because the high temperature required to melt the film (256.degree. C.) will also damage it. Numerous adhesive coatings have been applied to polyester film in order to improve its heat sealability and a number of these coated films are commercially available.
An example of heat-seal-coated polyethylene terephthalate film is disclosed in D. Hedge, "Heat Seal Coated Polyester Films" British patent specification No. 1,078,813 (Aug. 9, 1967). The Hedge '813 patent discloses a process for the production of a heat seal coated, biaxially oriented polyethylene terephthalate film comprising the steps of melt extruding a substantially amorphous polyester film, drawing the film in the longitudinal direction, thereafter applying a heat seal coating to one or both sides of said film and then drawing the coated film in the transverse direction (page 1, lines 36 to 44). The heat seal coating may be applied as a melt, as a solution in an organic solvent, or as an aqueous dispersion. Polymers which may be conveniently applied to the film in the molten state as a heat seal coating include polyethylene and polypropylene. Polymeric coatings which may be applied as solution or as aqueous dispersions include vinylidine chloride polymer and copolymers of vinylidine chloride with one or more of acrylonitrile, itaconic acid or methacrylic acid, polyvinyl acetate, partially hydrolyzed polyvinyl acetate, co-polymers of vinyl chloride and vinyl acetate, butadiene/acrylonitrile copolymers, butadiene/styrene copolymers, butadiene/methyl methacrylate copolymers, butadiene/methyl methacrylate/styrene copolymers, methyl methacrylate/methacrylic acid copolymers, copolyesters of terephthalic acid and another dicarboxylic acid with a glycol, e.g. those containing not more than 4.0 molecular proportions of combined terephthalic acid to one molecular proportion of combined sebacic acid; copolymers of vinylidene chloride and vinylchloride or alkyl acrylates, copolymers of vinyl acetate with vinyl chloride, copolymers of vinyl acetate with ethylene and copolymers of vinyl chloride with ethylene. Heat seal coatings make it more difficult to recycle polyethylene terephthalate (PET) film because the coating contaminates the polymer. In addition, the thermoplastic coating may preclude the use of the coated polyester film in food packaging applications under the applicable Food and Drug Administration regulations, 21 C.F.R. 177.1630 et seq.
The prior art has also employed terephthalate comonomers such as isophthalic acid to produce copolyesters which are more amenable to heat sealing than polyethylene terephthalate homopolyester. The copolyester may comprise the polyester film itself, or a second copolyester film may be laminated onto a first polyethylene terephthalate homopolyester film. An example of a copolyester/polyester laminate is disclosed in P. Stainer et al, "Polyester Film Composites," European patent application No. 81300643.4 (published Sept. 16, 1981). The Stainer application discloses a polyester film composite comprising an oriented layer of a first linear polyester and a heat sealable secondary layer, adherent to the primary layer, comprising an essentially amorphous second linear polyester, said secondary layer containing from .005 to 10% by weight, based upon the weight of the second linear polyester, of a finely-divided particulate additive having an average particle size greater than the thickness of the secondary layer dispersed substantially uniformly throughout the secondary layer, the exposed surface of the secondary layer having anti-blocking surface protrusions produced by the particulate additive, said surface protrusions being present in an area concentration of at least 50 protrusion peaks per square millimeter of surface and having a peak height of at least 0.5 micrometers measured from the surface of the polymer. Polyethylene terephthalate primary layers are particularly preferred. The second linear polyester of the second layer is preferably a copolyester derived from one or more of terephthalic acid, isophthalic acid and hexahydroterephthalic acid and one or more glycols, preferably ethylene glycol. The secondary layers may be disposed on one or both sides of the primary layer. The film composites may have a total thickness in the range of 10 to 500 micrometers and the secondary layers preferably have a thickness of up to 10 micrometers. The particulate-containing polyester composites disclosed in the '835 application were compared to non-particulate containing comparative examples which comprised polyethylene terephthalate as the first linear polyester in the primary layer and a secondary layer comprising a copolyester of 82 mole percent ethylene terephthalate and 18 mole percent ethylene isophthalate.
Unfortunately, there are processing drawbacks associated with the use of copolyester films, especially in reclamation of scrap polyester film.
A third solution proposed by the prior art involves surface modification of biaxially oriented polyethylene terephthalate film by flame, electric corona, or ultraviolet radiation treatment. For example, J. Cristie et al, "Oriented Heat Sealable Polyethylene Terephthalate Plastic Films," British patent specification No. 1,117,354 (June 19, 1968) discloses oriented and heat set polyethylene terephthalate films possessing a surface layer of amorphous disoriented polymer. Such film may be produced by a process in which a drawn and heat set polyethylene terephthalate film is caused to pass or travel over a cooled or rotating roller so that one surface of the film is maintained in contact with said roller and a localized area of the opposite surface of the film is simultaneously subjected to a flame of intense heat, the rate of travel of the film being such that the period of residence of every point in the heated area of the film is sufficient to melt the surface of the film at that point but insufficient to distort the film. Biaxially oriented films which have been surface modified in this manner do not suffer substantial loss of tensile strength. The '354 reference discloses that films produced by this process may be sealed to one another by pressing them together at a temperature between 80.degree. C. and the melting point of the polyethylene terephthalate, preferably at a temperature not higher than a temperature of 30.degree. C. below the melting point of the polyethylene terephthalate, and below the temperature at which the film has been heat set. The higher the temperature for heat sealing the more quickly can it be accomplished. The preferred temperature for heat sealing is between 120.degree. and 180.degree. C. Such polyethylene terephthalate films may be heat-sealed together at their edges to form articles such as bags. The Christie reference states that the amorphous surface of the biaxially oriented polyester enhances the film's resistance to tear and crack propagation.
The Cristie flame process requires close control over process parameters such as burner position, film speed, flame temperature and flame height. Even with such control, it is believed that such surface-modified biaxially oriented polyester film may suffer from non-uniformity in the surface modification. For example, if the flame becomes momentarily more intense the entire polyester film, rather than just the surface, might lose its orientation, melt, or even begin to burn. Conversely, if the flame momentarily becomes less intense the film surface may be unaffected by the process entirely. Certain other difficulties typically encountered with flame treatment of polymeric film are discussed in W. Bryan, "Flame Treatment Of Polymeric Film And Apparatus," U.S. Pat. No. 3,145,242 (Aug. 18, 1964).
"Subbing Process For Photographic Films," British patent specification No. 1,149,812 (Published Apr. 23, 1969) discloses UV irradiation of biaxially oriented, polyester film to enhance the adhesion of the film to photographic emulsion coatings. The '812 patent requires the application of a solution of gelatin or hydrophilic resin in a mixed organic solvent containing 1-25 percent by weight of a swelling agent. The wavelength of the ultraviolet radiation may range from 1800 to 3200 angstroms, and the irradiation preferably takes place either during or after the biaxial orientation process at a temperature ranging from 150.degree. to 250.degree. C.